Factors Affecting Pulmonary Capillary Oxygen Partial Pressure: Correct Answer Explained

The correct answer is D. altitude.

The partial pressure of oxygen in pulmonary capillaries is increased by increasing the partial pressure of oxygen in the alveoli, increasing the pulmonary capillary blood flow, and increasing the respiratory membrane surface area. Altitude decreases the partial pressure of oxygen in the atmosphere, which in turn decreases the partial pressure of oxygen in the alveoli and the pulmonary capillaries.